Seizures in Afghanistan of ammonium nitrate fertilizer, the main explosive used in Taliban bombs, more than doubled in the first seven months of 2012 compared with the same period last year, U.S. officials said.

Despite that increase, senior U.S. officials said, the number of improvised explosive devices, or IEDs, manufactured with the chemical compound is on pace to surpass the record levels of 2011.

“We are sweeping ammonium nitrate fertilizer off the battlefield at historic rates,” said a senior U.S. official who spoke on the condition of anonymity to discuss sensitive intelligence. “But the IEDs are going up at historic rates, too, and it is directly related. It is a supply issue.”

The homemade bombs, which are most often planted along roads and footpaths, are a leading killer of U.S. and NATO troops in Afghanistan. The ammonium nitrate used as the explosive component is manufactured at two plants across the border in Pakistan, and officials said the manufacturer has resisted efforts to control the flow into Afghanistan.

Figures provided to The Washington Post show that U.S. and Afghan troops have seized about 480 tons of ammonium nitrate fertilizer this year, enough explosive material to manufacture 30,000 to 50,000 IEDs.

During the same period, U.S. and Afghan troops have either triggered or discovered 16,600 of the makeshift bombs, a slight increase over 2011. In June, U.S. and Afghan forces encountered 1,900 IEDs, a record number in a single month for the 11-year war.

“Unless we do something about the ammonium nitrate from Pakistan, we are going to continue to face these numbers and threats,” the senior U.S. official said.

Sen. Robert P. Casey Jr. (D-Pa.), a member of the Senate Foreign Relations Committee, has conducted several hearings and investigations into the smuggling of ammonium nitrate from Pakistan into Afghanistan. He has pushed for a tougher stance against Pakistan for failing to curtail the trade and is troubled by the lack of progress.

“One year ago this month, I met in Islamabad with senior officials who committed to comprehensively regulate the component materials of IEDs, including calcium ammonium nitrate,” Casey said in an interview. “Since then, there has been minimal progress. The administration will soon need to certify that Pakistan is addressing the IED threat in order to release millions in security assistance and, as of now, I cannot see how Pakistan will reach this threshold.”

The large number of IEDs uncovered this spring and summer, the traditional fighting season in Afghanistan, demonstrates that the Taliban insurgency in Afghanistan has remained resilient even as U.S. forces have increased in recent years and the territory controlled by insurgent forces has been reduced.

U.S. troop levels are set to shrink to about 68,000 by the end of September, from a peak of about 100,000 in 2011. But once U.S. forces have left, the massive increase in explosive material flowing in to Afghanistan could make it difficult for Afghan troops to hold territory seized from the Taliban in recent years.

Unlike their U.S. counterparts, Afghan forces lack sophisticated technology to find and clear buried bombs. As they have taken a more prominent role this year, Afghan troops have seen a 76 percent increase in IED attacks compared with the same period in 2011, U.S. officials said.

Although U.S. casualties from the bomb blasts are down — a sign that U.S. and Afghan troops are getting better at finding them — U.S. fatalities in Afghanistan have held steady in 2012.

Almost all of the ammonium nitrate used in the Taliban’s bombs comes from two big fertilizer plants in Pakistan, both owned by the Fatima Group, based in Lahore. The production and sale of ammonium nitrate is legal in Pakistan but banned in Afghanistan because of the IEDs.

Officials from the Fatima Group did not respond to a request for comment.

Most of the fertilizer produced by the Fatima plants is used by small farmers in Pakistan who depend on it for their survival, U.S. officials said.

The 110-pound bags of fertilizer are easy to spot, and they must be hidden in secret compartments to be hauled across the border in trucks. But converting the fertilizer into explosive material that is 1.4 times more powerful than TNT is fairly simple, requiring only water and a heat source.

After processing, the ammonium nitrate is white and powdery, and Taliban fighters on both sides of the border often package it as laundry detergent, making it difficult for Afghan officials to spot as it is being smuggled into and around the country.

Earlier this year, senior U.S. military officials met with Fatima Group executives to try to persuade them to add U.S.-supplied pink or yellow dyes to their fertilizer to make it easier to spot at border crossings. But Fatima Group officials rejected the American entreaties, according to U.S. officials.

“They said, ‘We are not going to do it because it would single us out . . . as being the source of the material,’ ” said the senior U.S. official, who met with Fatima executives and recounted the conversation.

U.S. officials have photographs of tens of thousands of pounds of the Fatima Group’s fertilizer that has been smuggled into Afghanistan in trucks and then confiscated by U.S. and Afghan troops. The company’s two multimillion­dollar plants in Pakistan’s Punjab province are the only facilities authorized by the Pakistani government to manufacture the fertilizer.

After allowing U.S. officials to tour one of the plants in 2011, Fatima Group executives cut off contact, saying all future communications with the company must be conducted through Pakistan’s Foreign Ministry, according to U.S. officials involved in the talks.

“There is an ISI link in this,” said the senior U.S. official, referring to Pakistan’s powerful Inter-Services Intelligence agency. “They put the clamps on us.”

Although the Fatima Group is a multinational company, U.S. officials have limited tools to put pressure on the business to help control smuggling of ammonium nitrate. The U.S. Commerce Department can place foreign companies on the Entity List, which prohibits U.S. firms from doing business with them.

The Fatima Group works with Bank of New York Mellon and Bank of America to facilitate trading of the company’s shares in the over-the-counter market, according to Fatima Group news releases posted on the company’s Web site.

To place a company on the Entity List, U.S. officials must be able to prove that it is knowingly selling its product to insurgent groups or terrorists. U.S. officials acknowledged that they have not been able to meet that threshold with the Fatima Group.

“You have to have a witting link,” the senior U.S. official said. “That is what is frustrating on this.”

Updated Sept. 27: Fatima Group, which operates two major fertilizer plants in Pakistan, was unable to respond to an Aug. 17 request for comments from The Post before the publication of this article on Aug. 18. In a recent letter, the company made several points that would have been included in the original article.

Fatima Group disputes the assertion by U.S. officials that most of the ammonium nitrate used in Taliban bombs comes from its plants. The company said other countries bordering Afghanistan have a much larger production of ammonium nitrate fertilizer than Pakistan and produce fertilizer that is much easier to convert into explosives. They also have more porous borders.

The company also said it has cooperated with the U.S. and Pakistani governments to curtail fertilizer smuggling into Afghanistan and that it discontinued direct talks with the U.S. military after the Pakistan government suggested to company officials that it wanted a single channel of communication.

The letter said the company’s anti-smuggling efforts included redesigning its ammonium nitrate fertilizer bags to make them more distinctive, adding numbers to the bags, stopping the sale of fertilizer in the tribal areas bordering Afghanistan and ensuring that the product is sold only by authorized retailers who keep formal records. The company said it declined to dye its fertilizer on the advice of the Pakistan government, which said the issue should be dealt with on a regional basis rather than by singling out Fatima Group.

“An impression is being created that there seems to be apathy on Pakistan’s part,” said the letter. “The fact is that ever since the issue of IEDs was raised unprecedented efforts have been underway at both the government level and in terms of our group’s cooperation.”